The invention relates to the general field of microanalysis based on focused ion beams, with particular reference to samples that are insulators.
Charging effects and thermal damage are typical problems in most samples examined by charged particle microscopy, when the current level is in the nanoampere or microampere range. These current levels severely degrade the image resolution and several methods have been developed to solve the problem. These include:
a. Coating a thin film of high conductivity/thermal conductive material on the specimen surface
b. Operation at low beam energy,
c. Incorporating a second beam of ion (for scanning electron microscope or SEM) or electrons (for focused ion beam or FIB) to discharge the specimens (charge compensation or charge neutralization). The ion or electron beam is called a flood gun. Particularly for the glass specimens such as TFTs (Thin Film Transistor) on glass, ultraviolet light is used to discharge the accumulated charge.
Coating techniques are the most popular of the above methods for charged-particle microscopy. Among the different coating techniques we may mention:
1. Surfaces coating: vacuum evaporation and sputter coating technique which are standard procedure in most electron microscopy and analytical laboratories.
2. Metal impregnation from fixative solutions of Os(osmium) and Mn(manganese), with or without the use of organic metal ligands or mordants such as thiocarbohydrazide, galloglucose, paraphenylenediamine, by exposing specimens to OS40 vapor or by bulk staining the specimens after fixation with metallic salts.
3. Spraying or impregnating with organic anti-static agents derived from polyamines, e.g. Duron, Denki, or sodium alk-benzene sulfonate, soaking in conducting colloids of noble metals or graphite or covering the sample with a thin (1-20 nm) polymer film such as Formvar or styrene vinylpryidine.
These operations are not suitable for samples which cannot be coated at the surface that is being examined, such as insulators, or samples for AES (Auger electron microscopy), SIMS (Secondary Ion Mass Spectrometry), and EDX (energy-dispersed spectroscopy of X-rays).
An earlier invention by the present inventor disclosed how the problem of charge and heat buildup on insulating specimens could be mitigated. In U.S. Pat. No. 5,474,803, Doong showed a method in which a rectangular cavity was formed directly underneath the sample area. The cavity wall closest to the sample was then coated with a conductive material so that electric charge and heat could leak to it for dissipation. Coating of this surface with the required material required that the substrate be accurately cut very close to the sample area to allow formation of and access to the cavity. Additionally, for two different sample areas that were close together, one would have to be sacrificed during the process of preparing the other.
A routine search of the prior art was performed and the following references of interest were also found:
In U.S. Pat. No. 5,977,543, Ihn et al. show a sample preparation method for charged particle microscopy. In U.S. Pat. No. 5,369,274 Brunger, and in U.S. Pat. No. 5,440,123 Ikeda, both show microscope methods, specifically substrate preparation.
It has been an object of the present invention to provide a structure that is suitable for the performance of microanalysis over a small area.
Another object of the invention has been that structure allow the microanalysis of insulating areas without the buildup of excess heat or electric charge.
A further object has been that structure make it possible to successively remove thin slices of the area under microanalysis.
A still further object has been to provide a process for the manufacture of said structure.
Still another object has been that said process enable the underside of a portion of the structure to coated with a layer of material even though there is another surface in close proximity to it.
These objects have been achieved by forming two cavities, on opposite sides of the area that is to be microanalyzed, that extend downwards into the substrate at an angle to its surface so that they intersect directly below the microanalysis area. The result is a cavity that is bridged by a beam having a triangular cross-section. Part of said beam is then selectively removed, resulting in a cantilever that extends out over the cavity with the microanalysis area located near its free end. A key feature is that the underside of the beam is coated with a layer of thermally and electrically conductive material. This is achieved by using a focused ion beam to first deposit the layer in question on the two lower sloping surfaces of the cavity. Then, as a result of sputtering by the ion beam itself, some of this material is ejected and redeposits on the underside of the beam.